Rotor and stator of an electric motor: definition, types, purpose

Sooner or later, a person interested in electrical engineering hears references to the rotor and stator, and asks the question: “What is this, and what is the difference between these devices? " In simple words, the rotor and stator are two main parts located in the electric motor (a device for converting electrical energy into mechanical). Without them, the existence of modern engines, and therefore most electrical devices based on them, would be impossible. The stator is a fixed part of the device, and the rotor is movable, they rotate in different directions relative to each other. In this article, we will analyze in detail the design of these parts and their principle of operation, so that after reading the article, the readers of the site Electrician himself there are no more questions left on this matter.

Content:

What is a rotor
What is a stator
Stator and rotor in asynchronous motors
Squirrel cage rotor
Phase rotor

What is a rotor

The rotor, also sometimes called an anchor, is a movable, that is, a rotating part in a generator or electric motors, which are commonly used in household and industrial equipment.

If we consider the rotor of a DC motor or a universal collector motor, then it consists of several main units, namely:

Core. It is made of many stamped thin metal plates, isolated from each other. a special dielectric or just an oxide film that conducts current much worse than pure metal. The core is collected from them and is a "puff cake". As a result, the electrons do not have time to accelerate due to the small thickness of the metal, and the heating of the rotor is much less, and the efficiency of the entire device is higher due to the reduction of losses. This design decision was made to reduce Foucault eddy currents, which inevitably arise during the operation of the engine due to the magnetization reversal of the core. This same method of dealing with them is used in AC transformers.
Windings. Copper wire coated with varnish insulation is specially wound around the core to prevent the appearance of short-circuited turns, which are unacceptable. The entire winding is additionally impregnated with epoxy resin or varnish to fix the windings so that they are not damaged by vibrations from rotation.
The rotor windings can be connected to a collector - a special block with contacts securely attached to the shaft. These contacts are called lamellas, they are made of copper or its alloy for better transmission of electric current. Brushes, usually made of graphite, slide over it, and at the right time an electric current is applied to the windings. This is called sliding contact.
The shaft itself is a metal rod, at its ends there are seats for rolling bearings, it can have threads or notches, keyways for attaching gears, pulleys or other parts that are driven electric motor.
A fan impeller is also located on the shaft so that the engine cools itself and does not have to install an additional device to remove heat.

It is worth noting that not every rotor has windings, which, in essence, are an electromagnet. Permanent magnets can be used instead, as in brushless DC motors. And in an asynchronous motor with a squirrel-cage rotor, there are no windings in the usual form, instead of them squirrel-cage metal rods are used, but more on that below.

What is a stator

The stator is the stationary part in the motor. Usually it is aligned with the body of the device and is a cylindrical part. It also consists of many plates to reduce heating due to Foucault currents, without fail varnished. On the ends there are seats for plain or rolling bearings.

The structure is called a stator pack and is pressed into the cast iron housing of the device. Inside this cylinder, grooves are machined for the windings, which, as well as for the rotor, are impregnated special compositions so that the heat is more evenly distributed throughout the device, and the windings do not rub against each other from vibration.

The stator windings can be connected in different ways, depending on the purpose and type of electrical machine. For three-phase motors, star and delta connection types are applicable. They are presented in the diagram:

A special junction box ("borno") is provided for making connections on the device body. In this box, the beginnings and ends of three windings are brought out and special terminal blocks of various designs are provided, depending on the power and purpose of the machine.

There are serious differences in the operation of motors with different winding connections. For example, when connected with a star, the engine will start smoother, but it will not be possible to develop maximum power. When connected with a delta, the electric motor will deliver all the torque declared by the manufacturer, but the starting currents in this case reach high values. The power grid may simply not be designed for such loads. Using the device in this mode is fraught with heating of the wires, and in a weak place (these are the joints and connectors), the wire may burn out and cause a fire. The main advantage of asynchronous motors is the convenience in changing the direction of their rotation, you just need to swap the connection points of any two windings.

Stator and rotor in asynchronous motors

Three-phase asynchronous motors have their own characteristics, the rotor and stator in them differ from those used in other types of electric motors. For example, a rotor can have two designs: squirrel-cage and phase. Let's consider the structural features of each of them in more detail. However, first, let's take a quick look at how an induction motor works.

A rotating magnetic field is generated in the stator. It induces an induced current on the rotor and thereby sets it in motion. Thus, the rotor always tries to "catch up" with the rotating magnetic field.

It is also necessary to mention such an important feature of an induction motor as rotor slip. This phenomenon consists in the difference between the rotor speed and the magnetic field generated by the stator. This is explained precisely by the fact that the current is induced in the rotor only when it moves relative to the magnetic field. And if the rotational speeds were the same, then this movement would simply not occur. As a result, the rotor tries to "catch up" in speed with the magnetic field, and if this happens, then the current in the windings ceases to be induced and the rotor slows down. At this moment, the force acting on him grows, he begins to accelerate again. This is how the effect of speed stabilization is obtained, for which these electric motors are in great demand.

Squirrel cage rotor

It is also a structure consisting of metal plates that serve as a core. However, instead of a copper winding, rods or rods are installed there that do not touch each other and are short-circuited with each other by metal plates at the ends. In this case, the rods are not perpendicular to the plates, but directed at an angle. This is done to reduce the pulsation of the magnetic field and moment. Thus, short-circuited turns are obtained, from here and the name.

Phase rotor

The main difference between a phase rotor and a squirrel-cage rotor is the presence of a three-phase winding, laid in the grooves of the core and connected in a special collector with three rings instead of lamellas. These windings are usually star-connected. Such electric motors are more laborious in production due to the complexity of the design, but their starting currents are lower than squirrel-cage motors, and they also lend themselves better to adjustment.

We hope that after reading this article, you no longer have any questions about what the rotor and stator of an electric motor are and what their principle of operation is. Finally, we recommend watching a video in which this issue is clearly considered:

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